I. Field of the Invention
The present invention generally relates to a multiple power supply unit comprising a plurality of DC stabilized power supplies that provides a constant voltage in parallel to a single common load.
More particularly, the present invention relates to a multiple power supply unit comprising a plurality of DC stabilized power supplies, each power supply having improved sensitivity to an overcurrent state of output current to the single common load during operation.
II. Background and Material Information
FIG. 3 is a block diagram depicting the configuration of a conventional DC stabilized power supply unit, namely, a DC/DC converter 300.
DC/DC converter 300 comprises a DC voltage supply 1, a switching element 2, a transformer 3, diodes 4, 5, and 6, an inductor 7, an electrolytic capacitor 8, a sense resistor 9, and a load 10. Sense resistor 9 is connected between a negative electrode of transformer 3 and load 10. An overcurrent detection circuit 11 is connected to both ends of sense resistor 9. Overcurrent detection circuit 11 comprises an amplification circuit 12, a resistor 13, a comparator 14, and a Zener diode 15, and detects excess current based on a voltage across sense resistor 9. Amplification circuit 12 comprises an amplifier 16, and resistors 17 and 18. An output from overcurrent detection circuit 11 (hereinafter referred to as an "overcurrent signal") is inputted to an alarm latch circuit 19. When receiving an output from alarm latch circuit 19 (hereinafter referred to as an "alarm signal"), a voltage control circuit 20 controls switching element 2 to stop generating pulses.
Next, the operation of overcurrent detection circuit 11 will be described. An amplified voltage across sense resistor 9, which corresponds to an output current to load 10, is compared with a reference voltage V.sub.Z, by comparator 14. The reference voltage V.sub.Z is a value used to determine whether the output current is in a state of overcurrent. That is, when the voltage across sense resistor 9 exceeds the reference voltage V.sub.Z, the output current is determined to be in a state of overcurrent.
When detecting the state of overcurrent, overcurrent detection circuit 11 outputs the overcurrent signal to alarm latch circuit 19. Alarm latch circuit 19 holds the overcurrent state and releases the state when it receives a release signal from another circuit (not shown in FIG. 3). While alarm latch circuit 19 holds the overcurrent state, voltage control circuit 20 outputs the alarm signal which causes switching element 2 to switch off. When the input is cut off in this way, DC/DC converter 300 shifts to a state in which it provides no output.
Recently, an electronic device in which two DC/DC converters are connected in parallel to a single common load to achieve improved reliability, has been provided. In this device, even when one of the DC/DC converters fails, the other continues to provide a load with a constant voltage. Therefore, this device can be used in a computer system which requires continuous operation, such as a non-stop server computer.
FIG. 4 is an exemplary graph depicting the volt-ampere characteristic of two DC/DC converters. In this device, while both DC/DC converters provide power to the load (hereinafter referred to as "two converter operation") in parallel, the output current from each DC/DC converter is one-half of the value output when a single DC/DC converter provides power to the load (hereinafter referred as to "single converter operation"). However, in this case of both converters being provided as DC/DC converter 300, even if the output current is one-half during two converter operation, the reference voltage V.sub.Z is the same as during single converter operation. As a result, the sensitivity of each of the two DC/DC converters to an overcurrent state of output is deteriorated during double operation.
Therefore, there is a need for a high-reliability multiple power supply unit, which maintains its sensitivity to an overcurrent state of output even when two DC stabilized power supply units are connected in parallel to a single common load.